1. Field of the Invention
The invention relates to the apparatus and a method of starting the combustion process in a fixed geometry combustor of a large gas turbine engine utilizing a catalytic element for promoting relatively low temperature auto-combustion and relying on a high throughput velocity to prevent flashback of the combustion process upstream of the catalyst.
2. Description of the Prior Art
Combustion chambers for large gas turbine engines utilizing a catalyst for supporting catalytic combustion are presently being investigated because of their ability to provide a relatively low temperature combustion zone relatively free of NO.sub.x gases.
It is generally well known that various high temperature catalysts promote combustion under a wide range of ratios and temperatures of the air/fuel mixture etc., but, as a general rule, the leaner the fuel mixture the greater the temperature within the catalyst must be to support auto-ignition and sustained auto-combustion. (See for example U.S. Pat. No. 3,797,231). The catalyst can be heated to the elevated auto-combustion temperature in many various ways such as passing the combustion air/fuel mixture through a heat exchanger, then through the catalyst until the auto-ignition temperature is attained as shown in the above-identified patent. After the rotor shaft has been brought up to running speed, the elevated temperature of the discharge air of the compressor associated with the gas turbine engine is generally sufficient to maintain the air/fuel mixture and the catalyst at a temperature able to maintain the conditions within the combustor capable of continuous auto-combustion.
It is important, however, to make certain that the maximum temperature of the catalytic element does not exceed approximately 2300.degree. F. Above this temperature the catalytic material tends to fuse or flow, thereby decreasing its effective surface area necessary for the proper catalytic activity. For the most part, the temperature is regulated and maintained within acceptable limits by the relative richness of the air/fuel mixture. However, even with a desirable mixture, there can be occasional flashbacks wherein combustion occurs upstream of the catalytic element producing hot streaks in excess of 3000.degree. F. Flame detectors are normally employed to detect when this happens and reduce the fuel until the upstream combustion is extinguished.
To minimize such occurrence it is beneficial to have a sufficiently high velocity through the combustor, that prevents i.e., by blowing out, any flashback and maintains the combustion process within the catalytic reactor element. Thus, under operating conditions of a combustion chamber having a catalytic reactor therein a high throughput velocity is desirable.
Since such high throughput velocity, under cranking condition wherein the turbine is being started, tends to put a large volume of relatively cool air through the combustor, which is contrary to the heated condition desirable for rapidly heating the catalytic element, and also tends to extinguish any standing pre-heating flame within the combustor upstream of the catalyst, combustors having variable geometry wherein the air entry into the chamber can be regulated have been proposed. (See FIG. 1 of above-identified patent). However, in large gas turbines it is generally undesirable to have moving parts in the high temperature zones such as the combustor.
The present invention provides apparatus and a method for heating the catalytic reactor element in the combustion chamber of a large gas turbine engine to its auto-combustion temperature wherein the chamber has a fixed (i.e., non-variable) geometry providing a high throughput velocity to prevent flashback during normal continuous operation.